1. Field of the Invention
This invention relates to a probe for transmitting and receiving ultrasonic waves (hereinafter referred to as an "ultrasonic transmit-receive probe"), and more particularly to an ultrasonic transmit-receive probe which is adapted to measure the flow velocity and flow rate of moistened fluid of high temperature and high pressure such as steam flowing through a pipe utilizing ultrasonic wave.
2. Description of the Prior Art
A device has been developed and extensively used in a variety of fields which is adapted to propagate ultrasonic waves through gas flowing through a pipe in order to measure the flow velocity and flow rate of the gas by utilizing the variation in propagation velocity of the ultrasonic waves. Such a conventional device for measuring the flow velocity and flow rate of steam utilizing ultrasonic waves is typically constructed in such a manner as shown in FIG. 1. More particularly, the measuring device includes a pair of sockets 10 mounted on a wall of a pipe 12 in a manner to be obliquely opposite to each other at a predetermined angle. In FIG. 1, only one of the sockets 10 is shown for the sake of brevity. Reference numeral 14 designates an ultrasonic transmit-receive probe 14 of which one end is fixedly received in each of the sockets 10 by means of a gas seal unit 16. The conventional measuring device also includes a terminal box 18 connected to the other end of the probe 14, which is then connected to a controller (not shown). The probes 14 insertedly connected to the sockets 10 are adapted to alternately and repeatedly carry out the transmission and reception of an ultrasonic pulse signal controlled by the controller, so that the propagation time of the ultrasonic signal propagated across steam flowing through the pipe 12 may be measured to obtain the flow velocity and flow rate of the steam.
Such an ultrasonic transmit-receive probe 14 employed in the conventional measuring device, as shown in FIG. 2, conventionally comprises a probe housing 20 of a cylindrical shape; an oscillator 22 airtightly or sealedly received in one end of the probe housing 20 by means of an oscillator housing 24, an oscillator holder 26, spacers 28 and 30 and an O-ring 32; and a plug 34 having one end connected through a lead wire 36 insulatedly covered with a tetrafluoroethylene tube to the oscillator 22 and securely held in the probe housing 20 by means of a plug stopper 38 and the other end outwardly projected from the probe housing 14 so as to be inserted into the terminal box 18. Reference numeral 40 indicates an O-ring interposed between the gas seal unit 16 and the probe housing 20.
In the ultrasonic transmit-receive probe 14 constructed as described above, the oscillator 22 is airtightly sealed by means of the O-ring 32. However, if steam is the fluid being measured, the O-ring 32 is exposed to direct contact with the steam. As a result the O-ring maybe constantly subjected to settling, since steam is high in temperature, pressure and activity. Even when the O-ring 32 is formed of the most steam-resistant material, it cannot be full proof against steam having pressure of 10 kg/cm.sup.2 at a temperature of about 183.degree. C. Thus, settling of the O-ring 32 occurs in a short period of time, resulting in a failure in sealing.
Such a failure in sealing causes steam to enter the interior of the probe housing 20, resulting in various problems, such as a failure in the insulation of the oscillator 22, a failure in bonding between the oscillator housing 24 and the oscillator 22, hydrolysis of heat-resistant plastic such as polyimide, which forms the spacer 30 and plug stopper 38, and the like; so that a failure in the measuring of the flow velocity and flow rate of steam occurs in a short period of time. Accordingly, it is necessary to frequently replace the probe. Further, when hydrolysis of the plug stopper 38 leads to the leakage of steam, the terminal box 18 insulation fails. Thus, it is also necessary to often replace the terminal box 18.
Thus, the conventional ultrasonic transmit-receive probe soon deteriorates in operational reliability eventhough it is necessary for such a probe to operate stably over a long period of time.
Accordingly, it would be highly desirable to develop an ultrasonic transmit-receive probe which is capable of operating stably and effectively over a long period of time and also capable of being positively protected against leaked steam.